PROJECT SUMMARY This proposal seeks to solve a vital problem: how can Alzheimer?s disease (AD) related changes be identified early in disease so that pathology can be prevented? To address this problem, we focus on a key subcellular structure, the synapse. Synapses are excellent targets because: 1) their integrity is central to cognitive function, 2) they are sensitized to dysfunction by aging, and 3) they are affected early in AD and causal to cognitive decline. In particular, work by our group and others implicates an ancient immune modulator called complement in AD synaptic pathogenesis. However, it has been difficult to study complement mediated synaptic decline because synapses themselves are small and heterogeneous. To overcome these limitations, we have developed the retina as a system to dissect synaptic biomarkers in AD. Unlike the brain, the retina is well mapped, accessible, and genetically tractable even at the level of single synapses. Moreover, the retina is the site of clinically relevant AD pathology, and it can be assayed noninvasively over time. Using this system, we will identify complement driven changes in the molecular, synaptic, and functional properties of the retina to serve as preclinical biomarkers for AD progression. To achieve this, we have developed novel tools and approaches: 3D nanoscopic imaging, cell-specific sequencing that maintains spatial tissue information, and paired functional analysis of the retina and the brain. These studies will provide unparalleled insights into the structural and molecular regulators of synaptic integrity in AD. They will also lead to the identification of novel biomarkers for detecting AD progression that may ultimately be useful in preventing AD development.